Neuropsychologically relevant mental abilities are studied in healthy men at different ages, in patients with clinically-diagnosed Alzheimer's disease, and in adults with Down syndrome at different ages. Tests are administered to evaluate intelligence, memory, language, visual attention, visuoperceptive and visuoconstructive ability, and perceptual-motor speed. Age-related differences in general intelligence and visual memory in our sample of healthy men, ranging in age from 20 to 83 years, were found to be smaller than the differences reported in normative studies of non-health-screened adults. Visual memory and the discrepancy between verbal and visuospatial abillty were not correlated with regional cerebral metabolic rates for glucose (rCMRglc) as measured by positron emission tomography (PET) and 18-Fluorodeoxyglucose. In Alzheimer's disease the discrepancy between verbal and visuospatial abilities was found to be correlated with lateral asymmetry of cortical rCMRglc. Asymmetry of visual attention to the right and left sides of extrapersonal space was also related to lateral cerebral metabolic asymmetry. Older Down syndrome adults perform worse on mental abilities tests than do younger subjects. Immediate verbal memory appears to be less affected by age in Down syndrome than are other abilities. Correlation between clinical, factor and localization scales in the Luria-Nebraska Neuropsychological Battery and lobar rCMRglc demonstrated statistically significant relations between metabolic deficits in the parietal lobe and reduced neuropsychological function subserved by the parietal lobe, in subjects with mild-moderate Alzheimer's disease but not in healthy controls.